An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) system will hereinafter be described with reference to FIG. 1.
FIG. 1 is a conceptual diagram illustrating an E-UTRAN structure. The E-UTRAN has evolved from a legacy UTRAN, and basic standardization thereof is now being conducted by the 3rd Generation Partnership Project (3GPP). The E-UMTS system may also be called a Long Term Evolution (LTE) system.
The E-UTRAN includes one or more “eNode B(s)” or “eNB(s)”. The eNBs are connected through an X2 interface. Each eNB is connected to a User Equipment (UE) through a radio interface and is connected to an Evolved Packet Core (EPC) through an S1 interface.
The EPC may include a Mobility Management Entity (MME), a Serving-Gateway (S-GW), and a Packet Data Network-Gateway (PDN-GW). The MME may include UE access information or UE capability information, and this information is generally adapted to manage UE mobility. The S-GW is a gateway in which the E-UTRAN is located at an end point, and the PDN-GW is a gateway in which a Packet Data Network (PDN) is located at an end point.
Radio interface protocol layers between the UE and the network are classified into a first layer (L1), a second layer (L2), and a third layer (L3) on the basis of three lower layers of an Open System Interconnection (OSI) reference model well known to a communication-system field. The first layer (L1) provides an information transfer service using a physical channel. A radio resource control (RRC) layer located at the third layer (L3) controls radio resources between the UE and the network. For this operation, the RRC layer exchanges RRC messages between the UE and the network.
FIG. 2 illustrates a control plane of a radio interface protocol between a User Equipment (UE) and a UMTS Terrestrial Radio Access Network (UTRAN) according to the 3GPP wireless access network standard. FIG. 3 illustrates a user plane (U-Plane) of a radio interface protocol between a User Equipment (UE) and an E-UTRAN according to the 3GPP wireless access network standard.
A radio interface protocol includes a physical layer, a data link layer, and a network layer horizontally. Vertically, the radio interface protocol includes a user plane for transmitting data information and a control plane for transmitting a control signal (i.e., a signaling message). The protocol layers shown in FIG. 2 may be classified into a first layer (L1), a second layer (L2), and a third layer (L3) on the basis of three lower layers of an Open System Interconnection (OSI) reference model well known to the communication system. The UE and the E-UTRAN include a pair of such radio protocol layers, and are used to transmit data via an air interface.
A physical layer serving as the first layer (L1) transmits an information transfer service to an upper layer over a physical channel. The physical layer is connected to a Medium Access Control (MAC) layer serving as an upper layer over a transport channel. Data is transferred from the MAC layer to the physical layer or the transport channel, or is also transferred from the physical layer to the MAC layer. And, data is transferred between different physical layers over the physical channel. In other words, data is transferred from a transmitting physical layer to a receiving physical layer over the physical channel. The above-mentioned physical channel is modulated according to an orthogonal frequency division multiplexing (OFDM) scheme, so that the physical channel uses time and frequency information as radio resources.
The MAC layer of the second layer (L2) transmits services to a Radio Link Control (RLC) layer serving as an upper layer over a logical channel. The RLC layer of the second layer (L2) supports transmission of reliable data.
The RLC layer function may be implemented as a functional block contained in the MAC layer. In this case, the RLC layer may not be present. A Packet Data Convergence Protocol (PDCP) layer of the second layer (L2) performs a header compression function to reduce the size of an IP packet header having relatively large and unnecessary control information in order to efficiently transmit IP packets such as IPv4 or IPv6 packets in a radio interval with a narrow bandwidth.
The radio resource control (RRC) layer located at the lowest of the third layer (L3) is defined on a control plane only. In association with configuration, re-configuration, and release of radio bearers (RBs), the RRC layer controls the logical channel, the transport channel, and the physical channels. In this case, the above radio bearer (RB) is provided from the second layer (L2) to perform data communication between the UE and the UTRAN. If an RRC connection is located between the RRC layer of the UE and the RRC layer of the radio network, the UE stays in an RRC connected (RRC_CONNECTED) state. Otherwise, the UE stays in an RRC idle (RRC_IDEL) state.
There are a plurality of downlink transport channels for transmitting data from the network to the UE, for example, a broadcast channel (BCH) for transmitting system information, a paging channel (PCH) for transmitting paging messages, and a downlink shared channel (DL-SCH) for transmitting user traffic or control messages. Traffic or control messages of a downlink multicast or a broadcast service (Multimedia Broadcast/Multicast Service: MBMS) may be transmitted over a downlink multicast channel (MCH). In the meantime, there are a plurality of uplink transport channels for transmitting data from the UE to the network, for example, a random access channel (RACH) for transmitting initial control messages, and an uplink shared channel for transmitting user traffic or control messages.
A plurality of logical channels are located above the transport channel, and are mapped to the transport channel. For example, the logical channels may be a broadcast channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).
A physical channel includes a plurality of subframes on the time axis and a plurality of subcarriers on the frequency axis. Here, one subframe includes a plurality of symbols on the time axis. One subframe includes a plurality of resource blocks and one resource block includes a plurality of symbols and a plurality of subcarriers. Each subframe can use specific subcarriers of a specific symbol (e.g., a first symbol) of the subframe for a Physical Downlink Control Channel (PDCCH) (i.e., an L1/L2 control channel). Each subframe has 0.5 ms. A Transmission Time Interval (TTI), which is time unit during which data is transmitted, is 1 ms.
In order to disclose detailed operations of the RRC state and the RRC connection method, a UE's RRC state and its RRC connection method will hereinafter be described in detail. The RRC state indicates whether a UE's RRC layer is logically connected to an E-UTRAN's RRC layer. If it is determined that the UE's RRC layer is logically connected to the E-UTRAN's RRC layer, this state is called a RRC connected (RRC_CONNECTED) state. If the UE's RRC layer is not logically connected to the E-UTRAN's RRC layer, this state is called a RRC idle (RRC_IDLE) state. A UE in the RRC connected (RRC_CONNECTED) state has a RRC connection, such that the E-UTRAN can recognize the presence of the corresponding UE in units of a cell. As a result, the UE can be effectively controlled. Otherwise, a UE in a RRC idle (RRC_IDLE) state cannot be recognized by the E-UTRAN, but is controlled by a core network (CN) in units of a tracking area larger than the cell. In other words, only the presence or absence of the above RRC-connected UE is recognized in units of a large region. If the RRC-connected UE desires to receive a general mobile communication service such as a voice or data service, the UE must enter the RRC connection state. Associated detailed description will hereinafter be described in detail.
If a user initially powers on his or her UE, the UE searches for an appropriate cell, and stays at a RRC_IDLE state in the searched cell. The UE staying at the RRC_IDLE state establishes a RRC connection in association with the E-UTRAN's RRC layer through a RRC connection procedure when it needs to establish the RRC connection, such that it is shifted to the RRC_CONNECTED state. The UE under the RRC_IDLE state must establish the RRC connection due to a variety of reasons. For example, if uplink data transmission is needed due to a user's phone call attempt, or if a paging message is received from the E-UTRAN such that a response message to the paging message must be transmitted, the UE under the RRC_IDLE state needs to connect the RRC connection.
A Non-Access Stratum (NAS) layer located above the RRC layer performs functions such as session management and mobility management.
In order to manage UE mobility, an EPS Mobility Management-REGISTERED (EMM-REGISTERED) state and an EMM-DEREGISTERED state are defined in the NAS layer. The EMM-REGISTERED state and the EMM-DEREGISTERED state are applied to a UE and a Mobility Management Entity (MME). The user equipment (UE) is initially in the EMM-DEREGISTERED status, and carries out an ‘Initial Attach’ procedure to access a network, such that it is registered in the corresponding network. If this ‘Attach’ procedure has been successfully carried out, the UE and the MME enter the EMM-REGISTERED state.
In order to manage a signaling connection between the UE and the EPC, an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. The above-mentioned states are applied to the UE and the MME. The UE in the ECM-IDLE state is in the ECM-CONNECTED state when it establishes an RRC connection with an E-UTRAN. If the MME of the ECM-IDLE state makes an S1 connection with the E-UTRAN, it enters the ECM-CONNECTED state. If the UE is in the ECM-IDLE state, the E-UTRAN has no context information of the UE. Therefore, the UE of the ECM-IDLE state carries out an UE-based mobility procedure (e.g., cell selection or cell reselection) without receiving a command from the network. Otherwise, if the UE is in the ECM-CONNECTED state, UE mobility is managed by the network. If the UE is in the ECM-IDLE state and the UE's location recognized by the network changes to another UE location, the UE performs a Tracking Area Update procedure, such that it informs the network of the UE's location.
System information will hereinafter be described in detail. The system information includes requisite information that must be recognized by the UE that desires to access a base station (BS). Accordingly, the UE must receive all the system information before accessing the BS, and must always include the latest system information. In addition, the system information must be recognized by all UEs contained in one cell, such that the BS periodically transmits the system information.
The system information is classified into a master information block (MIB), a scheduling block (SB), a system information block (SIB), etc. The MIB includes physical configuration information (e.g., a bandwidth) of the corresponding cell. The SB includes transmission information such as a transmission period of each SIB. The SIB is an aggregate (or a set) of mutually-associated system information. For example, a certain SIB includes only information of a neighbor cell, and a certain SIB includes only information of an uplink radio channel used in the UE.
The service provided from the network to the UE may be classified into three types. The UE may differently recognize a cell type according to categories of received services.
There are a variety of service types, i.e., a limited service, a normal service, and an operator service.
The limited service may provide an emergency call and an Earthquake and Tsunami Warning service (ETWS), and may be received from the acceptable cell. The normal service may indicate a public use of a general purpose, and may be received from a suitable cell. The operator service may indicate a service for a communication network enterprise and may be utilized only by the communication network enterprise, and it is impossible for a normal user to use this operator service.
In association with the type of a service provided from a cell, there are a variety of cell types, i.e., an acceptable cell, a suitable cell, a barred cell, and a reserved cell.
The acceptable cell can provide the limited service to the UE. From the viewpoint of a UE, the acceptable cell is not barred and satisfies a cell selection reference.
The suitable cell can provide a normal service to the UE, and can also satisfy additional conditions simultaneously while satisfying the condition of the acceptable cell. A detailed description of the additional conditions is as follows. That is, this cell must belong to a Public Land-Mobile Area (PLMN) capable of accessing a UE, and the prevention of the execution of a tracking area (TA) update procedure of the UE need not be applied to this cell. If it is assumed that the corresponding cell is a Closed Subscriber Group (CSG), it is necessary for the UE acting as a SCG member to access the above-mentioned cell.
The barred cell is a cell that broadcasts information indicating the barred cell through system information. The reserved cell is a cell that broadcasts information indicating the reserved cell through system information.
In order to allow the LTE-advanced (LTE-A) system to further extend a bandwidth as compared to the LTE system, a carrier for use in a legacy LTE system is defined as a component carrier (CC), and a new technology for combining a maximum of 5 CCs into one group is now being intensively researched. Likewise, the technology for combining a plurality of component carriers (CCs) and using the combined CCs is called a carrier aggregation (CA) technology.
In order to maintain a communication link quality between a UE and a cell that provides a service to the UE, the UE continuously performs measurement. In particular, the UE determines whether a communication link quality with a cell that currently provides a service is in a communication unavailable state. If the cell that currently provides a current service is in a communication unavailable state, the UE declares a radio link failure (RLF). If the UE declares the RLF, the UE abandons maintenance of a communication state with the cell, and selects a cell through a sell selection procedure such that it attempts to reconfigure RRC connection with the selected cell.
When the UE starts the RRC connection reconfiguration procedure, the UE has to temporarily suspend the use of all radio bearers other than a ‘Signaling Radio Bearer (SRB) 0’. The SRB is a radio bearer that is used only to transmit an RRC message and a NAS message, and is classified into SRB0, SRB1 and SRB2. SRB0 is used for an RRC message that uses a Common Control Channel (CCCH) logical channel.
If the UE has successfully established the RRC connection reconfiguration, the use of an SRB1, the use of which is temporarily prevented, is resumed. After that, the RRC connection reconfiguration procedure is carried out so that Data Radio Bearers (DRBs) are reconfigured and data transmission/reception is resumed through DRBs.
According to the conventional art, if the carrier aggregation (CA) is used, configuration information of the carrier aggregation (CA) is contained only in the RRC connection reconfiguration message, such that it is impossible to use a plurality of component carriers (CCs) and it is possible to transmit and receive data through only one component carrier (CC). As a result, according to the conventional art, if a radio link failure (RLF) occurs in a HE which transmits and receives data using a plurality of component carriers (CCs), a predetermined time delay unavoidably occurs until data transmission/reception is resumed through a plurality of component carriers (CCs).